1. Field of the Invention
This invention relates to the modification of a surface by employing an electrical discharge in a gaseous atmosphere and, more specifically, to enhancing the adhesion, wettability, etc., of such a surface by exposure to an electrical discharge in a mixture of gases.
2. Description of the Prior Art
Processes for the initiation of polymerization at the surface of a substrate and specifically for the deposition of a thin, uniform polymer film upon a surface by employing a plasma (an excited gas phase) generated from an electrical discharge in an atmosphere of gaseous organic monomer are well established in the art. Generally in these processes a single ethylenically unsaturated monomer such as styrene is introduced into a previously evacuated chamber and brought to a partial vacuum. The item to be coated, e.g., a continuously moving web of metal, textile paper, plastic film, rubber, etc., serves as one of the electrodes or is in close contact with an electrode and, usually, alternating current at several hundred volts is supplied thereto at low current density to produce an electrical discharge. This electrical energy is transferred to the gaseous medium primarily to form a plasma by ionization or other excitation of the gas phase molecules, which eventually leads to chemical changes in the monomer as bonds are broken and new ones formed. It is believed that ions, radicals and/or ion-radical fragments are thus formed, which recombine as they accumulate on the electrodes or other surfaces in contact with the plasma to form polymeric coatings thereon. Detailed descriptions of such processes may be found in the art, for example, U.S. Pat. Nos. 2,932,591; 3,057,792; 3,068,510; and 3,069,283.
Normally, surface modificatioon of a substrate through exposure to a plasma as described above is accomplished by using a single gaseous monomer, preferably one with some carbon-carbon unsaturation, which alone serves as the propagating species in the polymerization to form a homopolymer. Saturated aliphatic compounds alone have been considered very inefficient in such gas discharge reactions, showing generally unacceptable yields of polymer per kilowatt hour.
Particularly significant improvements have been reported in the hydrophilic (wetting) properties of a variety of substrates which have been subjected to two or more successive ionizing discharges using a different activated gas for each such discharge. See, for example, U.S. Pat. No. 3,477,902 and 3,600,122. In general, the discharge in the first gas of these multistep processes serves to flood the surface of the substrate with stable reactive sites, e.g., peroxide groups, which subsequently lead to polymerization reactions at the substrate surface upon discharge in the second gas.
Multicomponent gas phase systems similar to those of the present invention have been employed in connection with the synthesis of copolymers through high energy nuclear radiolysis reactions, as described in my U.S. Pat. No. 3,462,354. Such a system may comprise, for example, the three gaseous components X, A and B, where X is a rare gas molecule and A and B are reactant gas molecules. Under specified conditions, when the above mixture is irradiated with high energy nuclear radiation, e.g., gamma radiation, the rare gas X absorbs most of the radiation energy, thereby forming rare gas ions which then transfer charge to the reactant molecule A, ionizing the latter, and these ions react with the other reactant molecule B in an ion-molecule reaction to form products of valve from relatively low value reactants. An illustrative system of the above-described type would be one comprising xenon as X, ethane as A and carbon monoxide as B.
Where one desires only small amounts of polymer for coating or surface modification purposes, however, high energy radiation from electron accelerators or radioactive sources is not well suited because of the expense, complexity, potential hazard and relatively long irradiation exposures connected therewith. It would, therefore, be advantageous if meaningful surface modification of a substrate could be obtained by combining the convenience and economies of a low energy source such as the aforedescribed electrical discharge with the advantages of material cost and simplicity offered by the simple gaseous mixtures such as those just mentioned.